1. Field of the Invention
The present invention relates to a grey-scale LCD driver which is able to realize multi-gradation of a liquid crystal display device by varying a voltage applied to a liquid crystal, and more particularly, to a grey-scale LCD driver which is able to realize multi-gradation by shaping a rectangular-waveform voltage used for a driving voltage into a ramp voltage through an integrating circuit and sampling the ramp voltage at selected bit timings.
2. Description of the Prior Art
Generally, liquid crystal display (LCD) devices are display units which display images by controlling the amount of light transmission utilizing dielectric anisotropy of liquid crystals, and are widely used as display units of laptop personal computer, word processor, and portable television receiver.
In the structure of the LCD device, there are a simple-matrix structure which controls arrangement of liquid crystal materials sandwiched between two stripe electrodes with a voltage generated at intersections of the stripe electrodes formed in a matrix shape, and an active-matrix structure which improves contrast, drive duty, and multi-gradation by adding thin film transistors to the simple matrix LCD device as a switching means for driving.
As shown in FIG. 4, a thin film transistor (TFT) active-matrix LCD device comprises an upper substrate 31, a lower substrate 51, and a liquid crystal layer 72 sandwiched between the upper substrate 31 and the lower substrate 51.
The upper substrate 31 comprises a polarizer 36 attached on an upper surface thereof, a color filter 34 sequentially disposed on a lower surface thereof, and a common electrode 32 disposed on the color filter 34. The common electrode 32 is made of Indium Tin Oxide (ITO).
The lower substrate 51 comprises thin film transistors TFT, an insulating layer 54, and pixel electrodes 52 electrically connected to the thin film transistors TFT through contact holes. Aligning films 75a and 75b rubbed in a predetermined direction are disposed on the common electrode 32 and the pixel electrodes 52, respectively. Therefore, the liquid crystal molecules of the liquid crystal layer 72 are twisted according to the rubbing direction of the aligning films 75a and 75b.
In the TFT active-matrix LCD device having above-mentioned structure, the thin film transistors TFT are turned on by signals applied to gate electrodes of the thin film transistors TFT, which results in that electrical signals applied to drain electrodes are applied to the pixel electrodes 52 through source electrodes. And then, the liquid crystal molecules sandwiched between the pixel electrodes 52 and the common electrode 32 are twisted in a direction different from a polarizing direction, which results in that the pixels are displayed.
In the LCD device, recently, the research and development to realize full-color closed to a natural color have been conducted. For example, Korean Patent Publication No. 96-3961 discloses three methods for realizing multi-gradation.
These three methods are a voltage-level driving method which displays multi-gradation by applying different levels of voltages, a frame driving method which displays multi-gradation by changing a effective voltage by varying a time for applying voltages in an unit frame, and a complex method which is in combination of the voltage-level driving method and the frame driving method.
These various methods are based on digital data from a controller, and the digital data is converted to analog data, which controls the voltage applied to the liquid crystal and time for applying the voltages to the liquid crystal layer. This leads the LCD device to realize multi-gradation of a unit pixel.
Hereinafter, a conventional grey-scale LCD driver for realizing multi-gradation will be explained in detail with reference to FIG. 5.
Referring to FIG. 5, the thin film transistor TFT of TFT LCD device is turned on by scan signals Vg applied to the gate electrode and transmits electrical signals applied from a switching element Q1 of the grey-scale LCD driver to the source electrode through the drain electrode. At this time, since the source electrode of the thin film transistor TFT is electrically connected to the pixel electrodes 52 on the lower substrate 51 shown in FIG. 4, a liquid crystal capacitor is formed between the pixel electrodes 52 of the lower substrate 51 and the common electrode 32 of the upper substrate 31.
Further, maintaining capacitor (not shown) connected in parallel with the liquid crystal capacitor is formed separately on the upper and lower substrate, in order to remove residual images by compensating DC voltage level shift which degrades the display quality at still images.
The conventional grey-scale LCD driver comprises a data storing section for storing n bits of digital data A0xcx9cAn-1 assigned for producing a grey-scale in synchronization with a clock signal CLK of a shift register, a counter 20 for generating counting data B0xcx9cBn-1 in synchronization with the clock signal CLK, a comparator 30 for comparing the digital data A0xcx9cAn-1 stored in the data storing section 10 with the counting data B0xcx9cBn-1 supplied from the counter 20 and outputting a comparing signal Comp if the digital data A0xcx9cAn-1 are equal to the counting data B0xcx9cBn-1 , a ramp voltage generating section 50 for generating a ramp voltage, a transistor Q1 for supplying the ramp voltage to the drain electrode of the thin film transistor TFT connected to the liquid crystal capacitor C depending on a signal from a sampling control section 40, and a sampling control section 40 for controlling the transistor Q1 in accordance with a control signal supplied from a controller (not shown) in order to sample the ramp voltage. Where, the transistor Q1 serves as a switching element.
The counter 20 divides selected time of a line into the number more than the number of gradations and counts them.
Now, the operation of the above-mentioned conventional grey-scale LCD driver will be explained.
The comparing signal Comp supplied from the comparator 30 is at inactive status, that is, a low level during the digital data A0xcx9cAn-1 stored in the data storing section 10 are not equal to the counting data B0xcx9cBn-1 supplied from the counter 20, and then if the digital data A0xcx9cAn-1 are equal to the counting data B0xcx9cBn-1 supplied from the counter 20, the comparing signal becomes a high level and activates the sampling control section 40. At this time, the sampling control section 40 outputs the sampling signal at a bit timing by means of the control signal Crt Sig supplied from the controller (not shown) and turns on the transistor Q1.
Therefore, the ramp voltage outputted from the ramp voltage generating section 50 passes through the transistor Q1 and the thin film transistor TFT and is charged in the liquid crystal capacitor C. And then, if the comparing signal Comp becomes the low level, the transistor Q1 is turned off. Therefore, the operation for charging the liquid crystal capacitor C is stopped.
The sampling bit timing is determined in accordance with the digital input data A0xcx9cAn-1 , and the liquid crystal capacitor C is charged to the voltage having a selected level within levels of multi-gradation, and then the selected level of multi-gradation is displayed.
Namely, the voltage for displaying a desired level of gradation is determined in accordance with the bit timing to be sampled, that is, position of a slope of the surface of the waveform of the ramp voltage to be sampled.
The number of bits of the counter 20 determines the number of gradations, which are realized by means of the LCD device. For example, if the number of bits is 8, it is possible that 256 gradations are realized (28=256).
Conclusively, in order to display the number of gradation, the conventional grey-scale LCD driver controls time for charging the liquid crystal capacitor C in accordance with the value of the digital data A0xcx9cAn-1 . The charging voltage of the liquid crystal capacitor C is determined according to the ramp voltage that is varied according to the elapsed time and the charging time.
However, as mentioned above, since the conventional grey-scale LCD driver uses the ramp voltage as the driving voltage for multi-gradation, an output impedance of the ramp voltage generation section 50 is easily changed according to the sampled position of the ramp voltage, and a noise voltage is generated.
At this time, if the noise voltage is greater than the voltage sampled at a least significant bit of the digital bit A0xcx9cAn-1 , the gradation corresponding to the least significant bit is not displayed. Therefore, the lower bit having voltage less than the noise voltage is not displayed, and it is impossible to display a large number of gradations.
Further, during the fabrication of the LCD device, since the ramp voltage generating section 50 is connected to a plurality of column of the transistor Q1 of the grey-scale LCD driver, and the magnitude of the load is changed by the number of the transistor Q1, it is required that the design of the ramp voltage generating section 50 is adjusted when it is applied to the LCD having different resolutions.
Since the ramp voltage generating section 50 used in the conventional grey-scale LCD driver is sensitive to load changes, the conventional grey-scale LCD driver lacks in the compatibility with respect to the change of the resolution of the LCD device.
Further, the ramp voltage generating section 50 is fabricated by a precise complicate driving circuit. By this, if the ramp voltage generation section 50 is used, the fabricating cost of the products is increased, and when the ramp voltage generating section 50 is mounted on a liquid crystal display panel, the area occupied by the ramp voltage generating section 50 is increased.
The present invention has been made in an effort to solve the above-described problems.
Therefore, it is an object of the present invention to provide a grey-scale LCD driver which can reduce the production cost of a liquid crystal display(LCD) device as well as the area which is occupied by a ramp voltage generating section on a LCD panel by using a rectangular-waveform signal as a driving signal instead of a conventional ramp voltage generating section, and transforming the rectangular-waveform voltage into a ramp voltage by a simple circuit.
It is another object of the present invention to increase the number of bits to be sampled or solve conventional problem which is not able to accurately sample the number of bits as many as a specified number by reducing a noise generated upon sampling the least significant bit
Further object of the present invention is to provide a grey-scale LCD driver, which responds to load changes so that the converter can be compatible with LCD devices having the different resolutions by using a circuit for generating the rectangular-waveform voltage to accurately display the least significant bit even when the number of bits to be displayed is increased.
To achieve the above objects, the present invention provides a grey-scale LCD driver comprising a data storing section for storing n bits of digital data assigned for producing a grey-scale in synchronization with a clock signal of a shift register; a counter for generating counting data in synchronization with the clock signal; a comparator for comparing the digital data stored in the data storing section with the counting data outputted from the counter and outputting a comparing signal if the digital data is equal to the counting data to control a sampling switching means; a rectaiagular-waveform voltage generating means for generating a rectangular-waveform voltage signal applied to the sampling switching means as a driving voltage; an integrating circuit for shaping a rectangular-waveform voltage supplied through the switching means into a ramp voltage, the integrating circuit being composed of a liquid crystal capacitor and a resistor; and a discharging switch means for discharging charges accumulated in the liquid crystal capacitor according to a control signal of a controller.
The driving voltage is applied to a thin film transistor being turn on or off in accordance with the control signal of the controller.
The resistor according to the present invention may be between the thin film transistor and the liquid crystal capacitor, or between the thin film transistor and the sampling switching means.
Further, the discharging switch means comprises a thin film transistor, and is between the resistor and the liquid crystal capacitor.